Target objects are often embedded within disordered environments in many important in vivo applications such as bio-medical imaging, phototherapy, and optogenetics. In these applications, it is necessary to deliver light waves to a deeply embedded target object for efficient optical imaging, sensing, and light stimulation.
However, a random wave diffusion induced by multiple light scattering on the disordered environments drastically limits the ability to reach the target object. When waves are propagating inside scattering medium, the waves are spread in both space and time, and only a small fraction of the injected energy reaches the target object.
A simple solution for increasing an arrival distance would be to increase the injecting energy, but this solution will increase a background noise and induce unwanted damage to the sample. Thus, to extend a working depth of optical methodologies, it is necessary to develop a method that increase the efficiency of energy delivery to the embedded target object.
In the past decades, numerous studies demonstrated the control of light waves traversing a scattering medium. The underlying concept of the past decades is to control the wavefront of illumination light to tailor an interference of multiple-scattered waves. In a work by Vellekoop et al. “Focusing coherent light through opaque strongly scattering media (Opt Lett 32, 2309-2311, doi:Doi 10.1364/Ol.32.002309, 2007), it is verified that a light wave transmitted through a scattering layer was focused by the wavefront shaping of an incident wave. Afterwards, temporal as well as spatial focusing has been realized by using broadband light sources.
Meanwhile, adaptive optics is one of the most representative approaches concerning the wave control within a scattering medium. The adaptive optics corrects sample-induced phase retardations and single-scattered waves by using a wavefront shaping device. However, the adaptive optics can control only a tiny fraction of the internal waves for the deeply embedded target object because the single-scattered waves are orders of magnitude weaker than the multiple-scattered waves.
Time-reversal of the ultrasonically modulated light waves is another approach that enables focusing optical waves to the acoustic focus within the medium. However, the concept was demonstrated using the transmitted wave through scattering medium, but it need to use phase-conjugation of the backscattered waves to warrant its utility for in vivo applications.
Also, a feedback control of the wavefront to increase a fluorescence signal from fluorophores embedded within a scattering layer is another approach to enhance light energy delivery to the target object, but this requires labeling agents. In addition, the operation is not target specific since all the fluorophores within the sample are affected.